PROGRESS OF MICROSCOPICAL SCIENCE. 
27 
fragments which make up the coarser ash-rocks seem generally to 
consist of felstone, containing both orthoclase anti plagioclase crystals 
or fragments ; but occasionally there occur pieces of a more crystalline 
nature, with minute acicular prisms and plagioclase felspar. 6. In 
many cases the only tests that can be applied to distinguish between 
highly-altered ash-rock and a felstone are the presence of a bedded or 
fragmentary appearance on weathered surfaces, and the gradual passage 
into less altei’ed and unmistakable ash. 
In the fourth division of his jxaper the author described some of 
the lavas and ashes of Cumberland of Lower Silurian age. 
With regard to these ancient lavas the following was given as a 
general definition : The rock is generally of some shade of blue or 
dark green, usually weathering white round the edges, but to a very 
slight depth. It frequently assumes a tabular structure, the tabulae 
being often curved, and breaks with a sharp conchoidal and flinty 
fracture. Silica 59-61 per cent. Matrix generally crystalline, contain- 
ing crystals of labradorite or oligoclase and orthoclase, porphyritically 
imbedded, round which the small crystalline needles seem frequently 
to have flowed ; magnetite generally abundant, and augite tolerably 
so, though usually changed into a soft dark-green mineral ; apatite 
and perhaps olivine as occasional constituents. Occasionally the 
crystalline base is partly obscured and a felsitic structure takes 
its place. 
The Cumberland lavas were shown to resemble the Solfatara 
greystone in the frequent flow of the crystalline base, and the 
modern lavas generally in the order in which the various minerals 
crystallized out. In external structure they have, for the most part, 
much more of a felsitic than a basaltic appearance. In internal 
structure they have considerable analogies with the basalts. In 
chemical composition they are neither true basalts nor true felstones. 
In petrological structure they have much the general character of 
the modern Yesuvian lavas ; the separate flows being usually of no 
great thickness, being slaggy, vesicular, or brecciated at top and 
bottom, and having often a considerable range, as if they had flowed 
in some cases for several miles from their point of eruption. Their 
general microscopic appearance is also very different from that of 
such old basalts as those of South Stafford and some of those of Car- 
boniferous age in Scotland. 
On the whole, while believing that in some cases the lavas in 
question were true basalts, the author was inclined to regard most of 
them as occupying an intermediate place between felsitic and doleritic 
lavas ; and as the felstone lavas were once probably trachytes, these 
old Cumbrian rocks might perhaps be called Felsidolerites, answering 
in position to the modern Trachy-dolerites. 
A detailed examination of Cumbrian ash-rocks had convinced the 
author that in many cases most intense metamorphism had taken 
place, that the finer ashy material had been partially melted down, 
and a kind of streaky flow caused around the larger fragments. 
There was every transition from an ash-rock in which a bedded or 
fragmentary structure was clearly visible, to an exceedingly close 
