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`IH CONT~UTION6 TO ECONOAi1C GEOLOGY Ikes, which seems ~ race its focus in the area occupied by the stock Johuson,19G8b,p.1i31). Tbo ]:nst. Spnnish Penk stock of gr;mile porphyry and grnnodiorite orphyry intrudes the Cuchnrn folnnntion of );ocene age and older alimentary rocks of Paleocene anti Cretaceous age. Doming of these edimeutnry rocks is quite extensive along the southern and western anks of the peak. Fulls (1901, p. 4) states that an nugite-granite orphyry occupies the summit null the western f~lce of the I:nst pnnish Penk, and a granite pm•phyry forms the main mess of tho l;nst pnnish Peak and the ridge exten[ling northwestward from it. Ife elieved that. this rock grades upward into the ungite-granite por- hyry through a very narrow zone. I{nopf (1936, p. 1735-173G) lassificd IIills' nugite-granite porphyry as a grnndiorike porphyry, nd considered it intrusive into rho granite porphyry. I~nopf's con- ention is confirmed by the discovery by the author of n black fine- rnined bm•der fncies where thegi•nnodioriteporphyry has been chilled gninst the granite porphyry. 't'his fncies coup be trace[1 tlirough- ut the southeast quadrant of the peak. Tbc intrusive that netamorphosed the se[limenhu•y rocks in the ficulted block south- lest of the stock is the granite porphyry. A few thin dikes of gnbbro vnprophyre nnl basalt. cut Lbe F.nst Spanish Penk stock near the outhensteru margin. The granite porphyry stock that forms the three hills known as ~TOrth, Middle, nud Sont.h \Vhite Peaks is int.rucled into the Pierre role, the '11•ini[lu[l sandstone, the Vermejo formntion, the Itn[on ornuition, anti the lowm• part of the Poison Canyon formntion. Chere is no apparent louring due to the invasion of the magma in Ic sedimentary rocks next to the igneous mass. No dilres cut the Itrusive, Hall dikes of grundiorite porphyry, syenite porphyry, and yenodim•ite porphyry that. nre radial from 1Vest Spnnish Penk xtend westward only ns far ns the eastern nuu•giu of the \Vhite Peaks rock. Iloweccr, several dikes of gnbbro Innyunpbyre and diorite ~ol•phyry nnl micrndim•ite cut. older rocks directly west of the \Vhite 'eaksstock. Four small npophyses of lhbnt. may be n concealed stock of syeuite porphyry intrude the Poison Cuuyou formation along the axis of the ,tintro syncline about •k miles suulblhest of 'Iercio in the exh•emo outlnvesleru part, of Ihe'I'rinidad coal licld. These oH'shonts form Hall rounded hills, aul thu coahlcls are not. well exposed. '1'be nasiou of the magma dots not. seem to have warped or mefamor- Ihosed the snrrouuding scrliment;ny rocks. The Dike Mounhlin stock ofsyamdim•ile porphyry is uu the uorth- vest margin of the coal field, nud thus probably ntfects tbeeoal-bearing RF.SOURCE6 OF TIi1NIDAD COAL FIELD, COLORADO I49 • rocks. 1'he stock intrudes the ('uch;u~a formation near the :~f the La Vela syncline. The cmdact of the iulrusihe ihith the snrrmulding sedimenhu•y rocks is cohered, but there does not appear to be [louring or metnnrorphism of the enclosing rocks. ~\ eery striking Hall COIISI11CllO11S SR'RI'111 of dikes rluliate from Dilce Mountniu in all di- rections. The rocks of the [likes nre simihu• to the rock of the main iuta•usive, and are fncies of syeno[liorite. LACCOLITH The I31nck I-Tills Inccolith of syeno[liorite pm•phyry has been intruded between fhe Pierre shale and the Poison Canyon formation on the axis of the Greenhorn anticline nt the. north end of the coal field. The base of [he lnccolith is generally well exposed, and metamorphism and doming of the enclosing sedimentur}' rocks is evident. Bells of the Poison (5ulyou formntion arch over the southern part of the Illack Hills intrusive, but lun•e been eroded from the mass elselhhere. Several small, circnlm• isolated outcrops of igneous rock bordering the Rlnck Bills are thmlght to lie oH'shools of the main mass. IIills (19110, p, a) thought that the Dike 9lountuiu Iwd lilnck Hills in- trusive rocks were genetically related, but, nlthongh they :u•e nx•ga- scopically similnr, no sm•faco connection is visible. PLUOS Goeuuuer Butte is nn igneous plug of lathe that intrudes the Cuchnra fm•mlUion about 21/~ miles smdbwest of Ln \-eta. 1'he rock is dark gray and very fine grained. Knopf (19a1i, p. li7R-lii!1) concluded that Goeuuuer Butte is a volcanic neck, luul ileutified the rock ns Intitc. A small plug of microgr:ulite intrudes the Cucbnru formntion nbont 51/~ miles uort.henst of Lu \'eta. The rock is ligld gray to white and very flue grained. DI%ES \[nny dikes intrude Upper Cretncemts and Tertiary sedimentnr}' rocks throughout the Trinidnl coal Held except. south of the Yurga- toire ]livm• where there nre only n few slrort dikes. The dikes range in width from 1 to more than 100 font, an[1 extend for n maximum of almost, l~k miles. 'They nro generally more resistant to erosion Ihlw the rocks of the inh•uded formations, nud consequently fhe}' stnu[I as relatively straight vertical walls ns nnlch ns 10(1 feet nbohe the sur- ronndiug country. ] [owever, some of the more mnfic [likes heather more rapidly Lhan the intruded sedimentary rocks to form G•enches. Polygonal joiuls nm•uRll to fhe [lilcc walls nre couunon. At soma 500000 O-Ol-4