OF THE NOKTH-EAST OF FIFE. 427 



gations of pyroxene and magnetite occur scattered throuj^li the mass 

 (see Plate XIII. fig. 4), so as to cause the rock to approach the 

 structure which I have described under the name of '• glomero- 

 porphyritic " *. It is worthy of notice that where decomposition has 

 commenced in the rock, the enstatite and magnetite crystals in these 

 aggregations are always the first to exhibit the signs of change. 



Although both of these rocks have the general aspect of basalts, 

 yet, as olivine is absent from then, I follow the great majority of 

 continental petrographers in classing them with the pyroxene-ande- 

 sites. I believe that this course is practically more convenient than 

 that of extending the groups of basalt and dolerite by including in 

 them the larger part of the pyroxene- andesites. As the presence or 

 absence of quartz in a rock is regarded as distinctive of important 

 groups, so, it appears to me, may olivine be taken as an eminently 

 characteristic mineral. Olivine, indeed, may be regarded as even 

 more useful in distinguishing a rock-group than quartz ; for it ap- 

 pears to be in almost every instance one of the first minerals to 

 separate from a magma, and is almost always clearly individualized, 

 while the silica of quartz-trachyte or quart z-andesite may remain as 

 tridymite, or in some other finely divided and not easily recognizable 

 condition, and in many rocks the quartz is clearly of secondary origin. 



The third rock to which I have to call attention is that consti- 

 tuting the red, porphyritic fragments between Scroggieside Farm and 

 Tay Bridge (see Plate XIII. fig. 7). It has a specific gravity of 2'58. 

 The compact hornstone-like base is seen under the microscope to be 

 much decomposed, in some cases showing much separation of chalce- 

 donic silica, but in places traces of an original flow-structure can be 

 detected, and microlites of felspar abound. The porphyritic crystals 

 exhibit the characteristic lamellar twinning of plagioclase felspars, 

 with the extinction of oligoclase. A few crystals of biotite also occur 

 scattered through the rock. The microscopic examination fully con- 

 firms the conclusion arrived at by Mr. Durham, that the remarkable 

 " pitchstone-porphyry " is only a local variety of this red porphyritic 

 rock. Very similar mica- and hornblende-dacites are found at Lent- 

 rathen and other localities in Forfarshire. 



The details given by Mr. Durham prove that the rock in question 

 is in all probability of Lower 01d-Eed-8andstone age, and in any 

 case it cannot be younger than Carboniferous. Yet in parts of its 

 mass the glassy structure of the rock is perfectly preserved. I believe 

 that this is at present a unique instance in the case of a rock of such 

 great antiquity. 



The " pitchstone-porphyry " rock is of a dark grey, almost black 

 colour, and has a resinous lustre, occasionally passing to sub- 

 vitreous. White felspar-crystals are scattered abundantly through 

 the glassy mass. The specific gravity of the rock is very low, the 

 mean of a number of determinations made for me by Mr. A. V. 

 Jennings in the Geological Laboratory of the Xormal School of 

 -Science being only 2-31. The proportion of silica in the rock was 

 found by him to amount to r)7"21 per cent. 



* Quart. Journ. Geol. Soc. vol. xhi. (188(>) p. 71. 



