M. R. Thorpe — The Abajo Mountains, Utah. 387 



morphosed. Not only is this true of the lower, but also of 

 the upper contacts. 2. There is an entire absence of frag- 

 ments of the igneous rock in the overlying sediments. 3. 

 The strata which flank the mountain have unquestionably 

 been disturbed since their deposition. 4. The presence 

 of a floor in the Abajo group seems practically certain, 

 although it is not entirely capable of proof, unless, from 

 the two small areas where it is visible, it can be said to 

 be characteristic of the whole mass. 5. The northern 

 and northwestern sections of the group have been 

 intruded at different horizons in the sedimentary strata 

 than have the southern and eastern sections. 6. On the 

 crests of the laccoliths masses of sediments, over 100 feet 

 thick, dip at various angles from the horizontal and are 

 highly metamorphosed. 



C. Factors not closely associated with either the intrusion or its cover, but 

 which point to intrusion in the region. 



1. Distribution of the laccoliths in a group instead of 

 in a system. This is characteristic of all laccolithic intru- 

 sions and is found in the Henry, Carrizo, La Sal and other 

 mountains of this type as well as in the Sierra Abajo. 2. 

 There is an entire absence of tuffs, breccias, lava flows, 

 volcanic bombs, lapilli, etc., in fact all the phenomena 

 which are so characteristic of extrusion. In conclusion 

 all the phenomena are those of intrusion and no evidence 

 has been found pointing to extrusion in this area. 



Contact Met amor phism. 



Contact metamorphism has taken place in the Abajo 

 area to a very limited extent. The endomorphic effects 

 have been very slight. In some cases the igneous rock 

 becomes of finer grain near the contact; in others it is 

 more porphyritic, both of which phenomena are indicative 

 of more rapid cooling. There is no indication that new 

 minerals have been developed. The exomorphic effects 

 have resulted in baking and hardening the overlying and 

 other strata in contact with the laccolithic masses. The 

 sandstones have been changed to quartzites and the shales 

 to hornfels and argillites, but still retain all of their 

 original structures, such as crossbedding, bedding planes 

 and conglomeratic and granular structures. Almost no 

 mashing or distortion of the sediments in contact with the 

 igneous rock could be observed. This hardening of the 

 sedimentary strata has made them tougher than the igne- 



Am. Jour. Sci. — Fourth Series, Vol. XLVIIT, No. 287.— November, 1919. 



97 



