HAEDWICKE'S SCIENCE- GOSSIP. 



103 



be difficult to recognize, especially if Biotite ; often 

 we shall observe it as forming fairly-shaped hexagonal 

 crystals, and the polariscope will also help us to 

 know it by its thinly laminated structure, giving rise 

 to fine parallel striae on the surface of its crystals. 

 Its colours, also, when polarized will be duller than 

 those of the quartz, for which it might sometimes 

 be mistaken at first sight, should it be a light- 

 coloured mica ; and then, again, it will frequently be 

 found that when the prisms of the polariscope are 

 crossed the mica becomes perfectly opaque, its 

 sections having been formed across the optical axis. 

 But let us now look at the quartz. We shall ob- 

 serve that this quartz is generally not crystallized 

 in definite forms, as are the felspar and the mica; it 

 appears as a matrix which has been at some time or 

 other soft and so is penetrated by the other crystals, 

 the interspaces of which it fills up : this shows us at 

 once that it must fhave been solidified after them, 

 and so was unable to assume its regular forms. 

 This is a very remarkable fact, and helps us towards 

 the secret of the formation of the granite. We 

 know that quartz requires a higher temperature to 

 melt it than does either the felspar or the mica, and 

 so, had the granite been formed as are regular 

 volcanic rocks in the ordinary way of igneous fusion, 

 we should certainly have found that the quartz 

 would have crystallized before either the felspar or 

 the mica, and it would have been seen in definite 

 crystalline form, and its crystals would have inter- 

 fered with and penetrated those of the other mineral 

 constituents of the rock. Again, if we look care- 

 fully at the quartz with a moderately high power, we 

 shall see in it certain small cavities, and some of 

 these will be seen to contain a certain amount of 

 liquid, and also an air-bubble, which will move as 

 the specimen is moved. This liquid has been proved 

 to be water, and from the fact of its not entirely 

 filling the cavity we learn that a reduction of tern, 

 perature has taken place since the water was first 

 caught up by the quartz, causing the contents of 

 the cavities to contract. Sometimes we shall find 

 other cavities, which, instead of containing water, 

 contain small crystals, or even air only. Now, from 

 all these facts it appears tolerably certain that the 

 granite was formed under peculiar circumstances ; 

 it has never been such a purely molten rock as 

 is the lava of a volcano, which is poured out from 

 its crater to the light of day. We gather that it 

 was rather formed at great depths in the earth, 

 where it may have been partially melted, partially 

 subjected to the action both of water and of steam, 

 charged with various mineral substances, and sub- 

 jected to enormous pressure. What the original 

 condition of granite was we cannot tell : some have 

 gone so far as to think that it may have been that of 

 a sedimentary rock, which has been metamorphosed 

 by the forces just alluded to. But whatever the 

 primary state of granite may have been, it3 present 



condition shows it to belong undoubtedly to the 

 igneous class of rocks, but to have been formed 

 under conditions differing from those which have 

 given rise to lavas reaching the surface. As far as 

 can be'gathered, the granite rocks, as such, have 

 never seen the light of day until exposed by denu- 

 dation, &c. ; their origin was deep in the central 

 portions of ancient volcanoes, where, by partial melt- 

 ing and slow cooling, under intense pressure, and 

 in the presence of some water, the various minerals 

 came together and crystallized into granite. 



NOTES ON THE DIPTERA— IV. 



MtjscidyE [continued). 



O INCE our space is necessarily limited, we are 

 ^ compelled to pass by many flies which other- 

 wise we should like to describe. As the Helomy- 

 zides are a large sub-family, we will take another 

 example from them — namely, Tetanocera. Elies of 

 this and two or three other allied genera are re- 

 markable for their oddity. There are ten or eleven 

 species of Tetanocera tolerably common, so that it 

 is difficult to know which to choose for description. 

 At fig. 55 is drawn the head of Tetanocera marginata^ 

 which, although not so common as one or two other 

 species, is perhaps the oddest fly of the whole 

 genus. It is the form of the head and of the 

 antennae particularly which gives these flies their 

 peculiarity, — indeed, a Tetanocera may always be 

 recognized by its antennae, which are carried hori- 

 zontally, and always have the third joint more or 

 less pointed. In T. marginata the second joint is 

 much longer than the third. This is the charac- 

 teristic feature of the fly. Its general colour is 

 rusty, and its wings are speckled brown and white. 

 Another species which we frequently meet with is 

 T. cucullaria. This, too, as are all the common 

 species of Tetanocerae, is a rust- coloured fly. It is 

 not unlike a dung-fly in shape, but the head, instead 

 of being round, is somewhat flattened, and the face 

 is white. It may be distinguished from other Teta- 

 nocerae as follows : — The bristle of the antennae is 

 covered with short downy hairs, while its second 

 and third joints, unlike the antennae of T. marginata 

 are of equal length. The wings are transparent, 

 with a number of indistinct pale brown dots to- 

 wards the tip. But perhaps the commonest Teta- 

 nocera is T. Hieracii, which can generally be pro- 

 cured from marshy places. Its characteristic fea- 

 tures are these : the bristle of the antenna is 

 fringed with hairs, while the third joint is slightly 

 longer than the second. Its wings are dark brown, 

 covered with three transverse bands of transverse 

 spots, and the discal transverse vein undulates 

 slightly, but not so much as in fig. 30. 



The mouths of the Muscidae are all formed on 

 one type, which, however it may be varied, can be 



