200 THE GEOLOGICAL AGE OF CENTRAL AND WEST CORNWALL. 



In a few places, and notably at Boscolla, near Truro, the 

 crystals of mica are comparatively large and exceptionally 

 abundant, and the rock having become completely disintegrated 

 to a considerable depth, these mica crystals— being the least 

 destructible portions, may be scooped up in glittering handful s. 

 The greenish-yellow scales lying in the beds of streamlets have 

 more than once been mistaken for grains of gold. In other 

 places fragments of ordinary white vein quartz may be seen 

 imbedded, but this is not very common. Distinct crystals of 

 felspar or hornblende are rarely to be seen by the unaided eye, 

 but in a few places large porphyritically embedded crystals 

 have been observed — as in specimen I, from Grreenbank. 



Carbonate of Ume is generally present in the joints — and 

 occasionally in the cavities of the rock, but apparently to a less 

 extent than is the case with the Cumbrian rocks described by 

 Professor Bonney. We have never found zeolites present. 



Microscopic characters. — Mr. John Arthur Phillips was the 

 first to describe the microscopic appearance of these Cornish 

 mica-traps. His description runs as follows.*' "Under the 

 microscope thin sections are seen to consist of a nearly equal 

 mixture of quartz and felspar and brown mica, enclosed in a 

 felspathic base. The felspar is monoclinic (orthoclase), and the 

 quartz contains a few small gas cavities, but no well-defined 

 fluid-cavities containing bubbles were observed." 



This description is evidently that of a " minette-felsite " or 

 "kersantite," it is very exact as far as it goes, as might be 

 expected from so skilled an observer, but it appears that his 

 sections must have been prepared from specimens containing 

 somewhat more quartz than usual. f We would add to Mr. 

 Phillips's description the following remarks : — In nearly aU the 

 sections of undecomposed specimens small crystals of a strongly 

 dichroic mineral resembling hornblende are visible, and not 

 unfrequently these are accompanied by minute crystals of some 



* Q. J. G. Soc, 123 p. 337. 



t Minette-felsite, strictly speaking, should contain only orthoclase ; 

 " kersantite " only plagioclase. These rocks appear to contain both kinds of 

 felspar — the potash predominating occasionally, but ordinarily the soda, as will be 

 shewn hereafter. 



