Wadsworth.] 486 [November 7, 



It is not my purpose to discuss the general geology of this is- 

 land which is regarded as belonging to the Lower Silurian, for 

 the reader will find sufficient information for ordinary purposes in 

 the Transactions of the Geological Society of London, 1814 (1), 

 ii, 450-457 ; Macculloch's Western Islands of Scotland, 1819, n, 

 205-222; Geikie's Scenery and Geology of Scotland, 1865, pp. 

 214-218 ; Quarterly Journal of the Geological Society, 1861, 

 xvit, 221, etc. 



Several years ago there were placed in my hands a number of 

 specimens of the Jura rocks by Mr. A. H. Wheeler of San Fran- 

 cisco, Cah, who had collected them. Descriptions were then 

 written and given to him, and it is a revision of these descrip- 

 tions that it is proposed to present to this Society to-night. 



The main rock of the island, as before stated, is a quartzite, 

 varying in texture from fine to coarse and forming in places aeon- 

 glomerate. One specimen has a fine-grained gray groundmass of 

 quartz grains holding larger rounded grains of milky, vitreous* 

 and pink quartz. The rock closely resembles some of the indu- 

 rated portions of the Potsdam sandstone of Wisconsin, and under 

 the microscope it is seen to be composed principally of rounded 

 quartz and feldspar grains. The cementing material is partly sil- 

 ica, but chiefly a material composed of minute greenish micaceous 

 scales, apparently formed from the mud and feldspathic material 

 of the rock. The feldspar grains of the original sandstone in the 

 metamorphosis of this rock to quartzite, have been likewise more 

 or less transformed into a mass of like greenish scales. Some 

 feldspars, however, retain evidence of their triclinic nature. Be- 

 sides these there also occur orthoclase and plagioclase grains that 

 are as clear and transparent as the quartz, which from their gen- 

 eral characters I am inclined to regard as secondary products in 

 the rock, probably replacing some of the original feldspar grains. 

 The quartz grains are as a rule well rounded showing prolonged 

 attrition. This according to Sorby would indicate rather a wind- 

 blown than a waterworn sand. 1 Surrounding the quartz grains, 

 between them and the matrix, are bands of exceedingly minute 

 cavities appearing almost like black dust. These bands also 

 exist in portions of the quartz and in the adjacent matrix and 

 feldspars. They traverse the two latter without much regard for 

 1 The Monthly Microscopical Journal, London, 1677,xvn, 113-136. 



