376 L. V. Pirsson — Geology of Conanicut Island. 



rock. The Lion's Head dike is precisely similar, only as it has 

 been more highly pressed it shows this change in a higher 

 degree and this reaches its highest development in the outcrop 

 on S. point of Austin's Hollow. Here the rock is strongly 

 folded in the strata and as already mentioned it has a pro- 

 nounced schistosity. Also under the microscope it shows a 

 schistose structure and contains considerable white mica and 

 chlorite along with the broken material of the original rock, 

 and were it not that all grades of transition from the nearly 

 normal rock at Hull's Cove down to these forms can be 

 traced, it would be impossible to recognize its original compo- 

 sition. 



Phyllite. — It would be impossible to give here a minute 

 description of all phases of the unaltered sedimentary rocks 

 existing on the island. A pale greenish or grayish phyllite of 

 extremely fissile character, however, is by far the most com- 

 mon rock, and it was selected for analysis. Thin sections of 

 it from various places showed pretty nearly the same general 

 character. Between crossed nicols with a moderate power the 

 slice presents a dark gray " pepper and salt " appearance inter- 

 spersed with fine points of brighter polarizing mineral frag- 

 ments. This is due to its excessively fine grain. Under very 

 high powers this is resolved into a mixture of sericite, chlorite, 

 occasional fragments of feldspar (orthoclase and perhaps albite), 

 specks of hematite, scales of graphite and pieces of quartz. 

 Occasionally very minute but idiomorphic crystals of tourmaline 

 occur. The chlorite and mica, interwoven into a fine mesh 

 and strung out along the planes of schistosity, are with the 

 quartz the chief components. This is everywhere penetrated 



SiO, - .-. 56-36 



TiO" .-- -_. 0-97 



A1 2 3 -.. 22-78 



Fe 2 3 -. 1-04 



FeO 6-25 



MnO ._ 0-15 



MgO ... 2-40 



CaO none 



Na 2 V48 



K 2 U 3-91 



H 2 ,. 5-01 



Total . 100-35 



by swarms and clouds of the most minute rutile crystals. 

 Muscovite occurs at times in tabular crystals which in their 

 orientation bear no reference to the planes of schistosity. The 



